Let $X$ and $Y$ be sets: $X = \{4,13\}$ $Y = \{8,13,15\}$ What is the set $X \cap Y$ ?
Answer: Remember that $\cap$ refers to the intersection of sets. The intersection of two sets $X$ and $Y$ is the collection of elements which are in $X$ and also in $Y$ The members of a set must be unique, and the order doesn't matter. $X \cap Y = \{13\}$